#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
double PI=2*M_PI;
int main() {
    cin.sync_with_stdio(false);
    int n;
    cin>>n;
    double a[n];
    for (int i=0; i<n; ++i) cin>>a[i];
    sort(a, a+n);
    double a1[n];
    int len=n;
    int st=0, en=n-1;
    for (int i=0; i<len; ++i) {
        if (i%2==0) {
            a1[st++]=a[i];
        }
        else {
            a1[en--]=a[i];
        }
    }
    double tmp=sin(PI/n)/2;
    double ans=tmp*a1[0]*a1[n-1];
    for (int i=0; i<n-1; ++i) {
        ans+=tmp*a1[i]*a1[i+1];
    }
    printf("%0.3lf\n", ans);
    return 0;
}
